Mammalian immature eggs (oocytes) grow and develop in follicles within the ovary. Immature oocytes are metabolically coupled to somatic granulosa cells, which surround the oocyte and nature the development of the oocyte until ovulation. The oocyte depends on its association with its companion somatic granulosa cells not only to support its growth and development, but also to regulate the progression of meiosis.
Follicle development is driven by a complex interaction between proliferation, differentiation and atresia. Atresia of ovarian follicles is an important process, accounting for the loss of over 99% of oocytes. It has been demonstrated from both in vivo and in vitro studies that follicular atresia is through an active process of programmed cell death, referred to as apoptosis. At the cellular level, apoptosis is characterized by cytoplasmic and nuclear fragmentation, chromatin condensation, DNA fragmentation and phagocytosis.
Apoptosis can be initiated in at least four different cell compartments in follicular development (theca cells, granulosa cells, cumulus cells and in the oocyte itself). During early atresia in antral follicles, the cumulus cells and the oocyte remain apparently unaffected by the atretic changes primarily manifested as apoptosis in the mural granulosa cells and, at a later stage, in the theca cells. The mechanism by which oocytes and granulosa/cumulus cells interact to escape apoptosis is poorly understood.
Traditionally it has been thought that the role of the oocyte in follicle development is passive, and that follicle development and therefore oogenesis, is driven by external hormones. However, it now appears that oocytes also secrete factors that promote follicle development. This oocyte control of folliculogenesis appears to be extremely important, as demonstrated by the fact that altered expression of oocyte paracrine factors can have profound effects on oocyte maturation, follicle development and fertility.
As such, the evidence now suggests that oocytes appear to play an active role in regulating follicle growth, and consequently follicle development and fertility, by secreting paracrine factors that regulate fundamental control elements of follicular granulosa cell function.
Despite the critical importance of such oocyte-secreted factors in regulating granulosa cell development, there is currently very little information regarding the identity of the factors secreted by oocytes that are involved in granulosa cell development, and how the expression of such factors can be used to control folliculogenesis, oocyte maturation and fertility.
In addition, the current methods for controlling folliculogenesis, oocyte maturation and fertility both in vitro and in vivo are inadequate for many reasons. Accordingly, there is a need for new methods of controlling granulosa cell development, so as to control follicle development, oocyte maturation and fertility.
The present invention arises from the finding that the apoptosis of granulosa cells is modulated by BMP-15 (also known as bone morphogenetic protein-15; GDF-9B) and/or BMP-6 (bone morphogenetic protein-6). Accordingly, the present invention relates to a method and composition for modulating the apoptosis of granulosa cells, and to methods and compositions for modulating oocyte maturation, follicle development and female fertility.
A reference herein to a patent document or other matter which is given as prior art is not to be taken as an admission that that document or matter was known or that the information it contains was part of the common general knowledge as at the priority date of any of the claims.